Piston valve for fluid pressure pumps or motors



Nov. 27, 1962 n. FIRTH ETAL 3,055,953

PISTON VALVE FOR FLUID PRESSURE PUMPS OR MOTORS Filed April29, 1960 vl/VVENTOR ATTORNEY United States Patent ()fitice 3,065,953 Patented,Nov, 27, 1.962

Filed Apr. 29,1960, Ser. No. 25,781 Claims priority, application GreatBritain May 1, 1959 2 Claims. (Cl. 251321) This invention relates topositive displacement fluid pressure pumps or motors and aims atimproving their efficiency.

One source of, loss of efficiency in known. forms of hydraulic pistontype pumps and motors having push-rod valve gear is that thermalexpansion of the push-rods during working of the pump or. motor causes,variations in valve timing;

In a positive displacement hydraulic. fluid, motor or pump according tothe present invention, efficiency is improved by means of a push rodoperated valve in which the valve member proper is of tubular form. Theend of this tube is closed and constitutes the valve head which controlsthe opening and closing of the port in the cylinder, and the internalcavity terminates at its inner end in a rounded or part-sphericalseating. This is engaged by a similarly shaped end on the push rod, theradius of the latter being not greater than that of the seating.

The seating is preferably at approximately the same level as the workingedge of the valve head so that thermal expansion of the latter in theaxial direction has negligible effect on the timing of the port events.

Advantageously, the tubular valve member has an external land around thehead of the valve the under-edge of which constitutes the port controledge, the external land being so dimensioned that, so long as theinternal seating is at a level intermediate the limits of the land, anyaxial expansion of the push-rod is at least partially compensated byaxial expansion of the land in the opposite sense.

The cavity in the tubular valve member is conveniently flared outwardstowards the skirt so as to prevent metalto-metal contact between thevalve and the push-rod except at the seating, thereby reducing transferof heat to the push rod.

An improved positive displacement hydraulic motor according to thepresent invention is illustrated, by way of example, in the accompanyingdrawing which shows an axial section through a piston and its valvegear.

The motor shaft carries a main eccentric 1 on whose periphery bear theslippers 2 of the piston rods 3. Each piston rod has a part-sphericallittle end bearing 4 which works in a complementary little end seatingassembly 5 in the piston 6. The latter reciprocates in a cylinder 7 in acylinder block 8 having a detachable head 9 which defines, with thepiston 6, a compression space 10. Inlet and exhaust ports 11, 12respectively open into this space at each side, and each port iscontrolled by a respective piston valve 13, 14 operated by a push rod15, of less external diameter than the internal diameter of the tubularvalve, from a corresponding inlet or exhaust cam 16, 17 bolted to theeccentric 1.

The push rod 15 of each valve transmits the required valve motion fromits cam 16 or 17 through a tappet 18 in the form of a deep cup having arounded or part-spherical seating 19 and side wall 20 which is upwardlyand outwardly flared. As shown, this side wall 20 is coned, the coneangle being a few degrees. Each tappet slides in a bush 21 pressed intoa socket in the cylinder block 8.

The bottom end of the push rod 15 is complementarily rounded or ofhemispherical shape at 2.2, the radius of the surface being not.greaterand preferably slightly leSS;-:- than that of the seating 19, soas. to ensure self-alignment. of the rod. 15 and also to provide,- aminimum, of bearing surface area for the. transmissionv of heat. from,the tappet 1.8. to the rod, 1

The. piston valvel;3 (or. 1. is of gen r lly u u ar tort having, spacedupper and lower external lands 23, v 24. The reduced diameter sternportion 25 between the lands registers with. an inlet passage 26 or anoutlet passage 2'1, as the case may be. The lands 23, 24 form, pressure.fluid seals withthe. wall of a respective. valve bore 28 or 29, theupper land 23 serving also to open and close theinlet or exhaust port11, 12. The. length of the stem 25. between lands is such as to ensurethat: the passage 26, or 27 isnever cutotf during. the valve. stroke.

The head of the valve 13 or 14 is constituted. by an inverted thimble 30having a rounded or partrspherical internal seating; 31 with a flared orconed side wall 32. The seating 31, receives a complementarity roundedor hemispherical end 33 at the top of the push rod 15, this end being ofa radius not greater than-and preferably less thanthat of the seating31.

The thimble 30 is threaded into thetop end of the bore through thetubular valve 13 or Hand is locked in its position of adjustment bymeans of aflanged nut 34 which also forms the lower abutment for ahelical compression valve spring 35. The upper abutment for the spring35 is provided by a shouldered plug 36 which is slidable in an upperextension 37 of the respective valve bore 28 or 29 and bears against aninternal shoulder on a cap 38 threaded on the external wall of theextension bore 37.

When the motor is working, the valves 13 and 14 are continuouslyreciprocated through their push rods 15 and tappets 18, by therespective inlet and exhaust cams 16, 17. During operation of the motor,the temperature of the working parts rises, and in hitherto knownconstructions of motor the expansion of the push rods and valves hasbeen such as to vary the valve timing, with consequent loss ofefficiency. In the construction of motor according to the presentinvention, this disadvantage is minimised or eliminated for thefollowing reasons.

First, the radial clearance between the internal wall of the tubularvalve 13, 14 and the push rod 15 substantially insulates the latter fromheat generated in the valve wall.

Secondly, the small area of contact between the hemispherical ends 22,33 of the push rod 15 and the co-acting seatings 19, 31 of the tappet 18and thimble 30, respectively, substantially eliminates the conduction ofheat from these components to the push rod 15.

Thirdly, the valve timing is primarily affected by a change in the axialdistance between the crown of the rounded seating 31 and the lower edgeof the land 23, and since this distance can be made very short, thermalexpansion of the valve can be effectively neglected. In any case, anyexpansion of the push rod 15 which may occur will raise the crown of therounded seating 31 and will tend to compensate for thermal expansion ofthe head end of the valve. This compensating action takes place becauseof the positioning of seating 31 within the valve 13, 14, at the end ofthe axial cavity, wherein push rod 15 extends through valve 13, 14 pastthe lower edge of land 23 which is the working edge of the valve.Therefore expansion, due to temperature changes, of rod 15 relative tothe driving cam 16, 17 or tappet 18 is in a direction to raise valve 13,14. Since the larger mass of valve 13, 14 is in a position below thecontact area of seating 31 the major expansion of valve 13, 14 is in adownward direction, opposed to the expansion of push rod 15. Thus,compensating actions of expansion of push rod 15 and valve 13, 14maintain accuracy in valve timing in the present invention.

What we claim is:

1. A valve mechanism for a positive displacement bydraulic machinecomprising a valve body having a valve bore, a generally tubular valvemember reciprocable in said bore and having a head end and a skirtportion, an external port control land around said head end terminatingadjacent the skirt portion in a working edge controlling a port in saidvalve bore, a rounded seating within the valve member at the end of anaxial cavity therein extending through the skirt portion and past theposition of said working edge and terminating in the head portion, apush rod extending with radial clearance through said cavity and havinga rounded end engaging with said seating, and driving means operativelyengaging said rod whereby expansion in the direction of movement of saidvalve member and said rod due to temperature changes is relative to theposition of said driving means.

2. A valve mechanism for a positive displacement hydraulic machinecomprising a valve body having a valve bore, a generally tubular valvemember reciprocable in said bore and having a head end and a skirtportion, an external port control land around said head end terminatingadjacent the skirt portion in a working edge controlling a port in saidvalve bore, a rounded seating within the valve member at the end of anaxial cavity therein extending through the skirt portion and terminatingbeyond and adjacent to the position of said working edge, a tappetreciprocable co-axially with said valve member, a rounded seating insaid tappet co-axially opposed to the rounded seating in the valvemember, a push rod extending with radial clearance through said axialcavity in the valve member and having rounded ends engaging with saidrounded seatings in the valve member and tappet, and driving meansoperatively engaging said tappet whereby expansion in the direction ofmovement of said valve member and said rod due to temperature changes isrelative to the position of said driving means.

References Cited in the file of this patent UNITED STATES PATENTS

